For example, hydrofluoric acid is recovered from waste liquids containing this acid conventionally by (1) a process wherein fluoride ions are reacted with a calcium, magnesium or like compound to precipitate the ions as a fluoride (JP-A-89984/1983, JP-B-71197/1991, etc.), or (2) a process wherein fluoride ions in the waste liquid are captured by a device having an ion exchange membrane and an ion exchange resin in combination and electrically dialyzed in an electric field to recover hydrofluoric acid (JP-A-123606/1989, JP-A-130782/1989, etc.). However, the process (1) requires decomposition of the fluoride with an acid such as sulfuric acid when hydrofluoric acid is reproduced, necessitating a great apparatus for the filtration of the precipitate and the subsequent decomposition. The process (2) requires a large quantity of electric power, further necessitating the step of separating the recovered acid from other acid, for example, by evaporation or forming the fluoride of calcium, magnesium or the like.
Thus, the prior art has the problems of requiring an additional chemical for recovering hydrofluoric acid and a further treatment of the recovered substance foe separation, and consuming a large amount of electric power.
An object of the present invention is to obviate these problems and to provide a process for recovering volatile acids without necessitating additional chemicals and much electric power and without necessitating a separation or decomposition treatment after recovery.
Another object of the invention is to provide a process for treating waste water utilizing the above process.
The present invention provides a process for recovering a volatile acid charaterized by bringing a waste liquid containing the volatile acid into contact with an amine having a boiling point of at least 50.degree. C. to thereby cause the amine to ionically adsorb the acid from the waste liquid, and thereafter heating the amine to desorb the acid.
The present invention also provides a waste water treating process characterized in that a waste liquid containing a volatile acid is brought into contact with an amine having a boiling point of at least 50.degree. C. to thereby cause the amine to ionically adsorb the acid from the waste liquid.
The waste liquids containing a volatile acid and to be treated according to the invention include, for example, those containing an acid such as hydrofluoric acid, hydrochloric acid or nitric acid. These waste liquids may further contain sulfuric acid or like nonvolatile component, ammonium fluoride or the like. Such nonvolatile components, even if present, will not affect the volatile acid recovering treatment of the invention but reduce the adsorbing capacity of the amine, so that presence of nonvolatiles in a large quantity is not desirable.
Examples of useful amines having a boiling point of at least 50.degree. C. are anion exchange resins, polyamines, aliphatic amines, aromatic amines and nitrogen-containing heterocyclic compounds. When having a boiling point of below 50.degree. C., amines themselves will evaporate when heated, and such amines are therefore undesirable. Examples of anion exchange resins are those comprising a copolymer of styrene and divinylbenzene, polyolefin polymer or the like as the base material. Examples of polyamines are 1,8-diaminooctane, 1,1,4,7,10,10-hexamethyltriethylenetetramine, N,N'-bis(3-aminopropyl)-1,3-propanediamine, etc. Examples of aliphatic amines are hexacyclen, hexamethylhexacyclen, etc. Examples of aromatic amines are 2,3-diaminonaphthalene, 9,10-diaminophenanthrene, etc. Examples of nitrogen-containing heterocyclic compounds are 4,7-phenanthroline, carbazole, 29H, 31H-phthalocyanine, etc.
According to the invention, the volatile acid is brought into contact with the amine, for example, by passing the waste liquid of volatile acid through a column packed with the amine, or by contacting the waste liquid with the amine batchwise without using a column. The contact thus effected causes the amine to ionically adsorb the volatile acid. With the latter batchwise process, it is necessary to separate the amine from the waste liquid after adsorption and before the subsequent step of heating, whereas the amine need not be so separated off with the former column process.
The adsorbed volatile acid is released from the amine by heating. The amine is heated by a desired method. For example, it is heated as contained in the column or with hot air passed through the column. It is desired that the heating temperature be such that the amine remains unaffected by heat to the greatest possible extent, that is, not higher than the limit within which the amine retains its function. The amine is heated until the component released therefrom becomes neutral in pH value. The preferred heating time is usually in the range of 1 minute to 50 hours.
With the process of the invention, the heating temperature is usually about 50 to about 180.degree. C., preferably about 70 to about 130.degree. C.
The present process is usable for treating waste water to remove volatile acids therefrom even if the concentration of the acid is low and can not be recovered efficiently. More specifically, the waste water treating process is conducted, for example, by a system comprising two columns arranged in parallel, packed with an amine and each connected to waste water, acid recovery and steam lines. The two columns are operated alternately. While one of the columns is treating waste water, the other column is used for recovering (regenerating) the acid in preparation for the subsequent treatment. When saturated with the adsorbed acid by passing waste water, the column is changed over to the other column. The saturated column is heated as by passing steam therethrough to recover the acid. Waste water can be treated continuously by repeating the above procedure.